A spark plug for an internal combustion engine is attached to an internal combustion engine (engine) and is used to ignite an air-fuel mixture in the combustion chamber of the engine. Generally, a spark plug includes an insulator having an axial hole, a center electrode inserted into a front end portion of the axial hole, a terminal electrode inserted into a rear end portion of the axial hole, a metallic shell provided on the outer circumference of the insulator, and a ground electrode provided on the front end surface of the metallic shell and adapted to form a spark discharge gap in cooperation with the center electrode. A resistor is provided within the axial hole between the center electrode and the terminal electrode, for restraining radio noise generated in association with the operation of the engine, and electrically connects the two electrodes (refer to, for example, Japanese Patent No. 2800279).
Generally, the resistor is formed from a resistor composition composed of a conductive material, such as carbon black, and ceramic particles (e.g., glass powder). In the resistor, the conductive material is present in such a manner as to cover the surfaces of the ceramic particles; as a result, the conductive material forms a large number of conductive paths which electrically connect the two electrodes. Due to the formation of a large number of conductive paths, even when some conductive paths are damaged by oxidation or the like induced by an electrical load, a sharp increase in resistance can be effectively restrained.
Meanwhile, in recent years, a reduction in size (a reduction in diameter) has been required for spark plugs. In order to reduce the size (diameter) of a spark plug, a reduction in the wall thickness of the insulator may be considered. However, a mere reduction in the wall thickness of the insulator may accompany deterioration in withstand voltage and mechanical strength. Thus, in order to reduce the size of a spark plug while preserving a certain thickness of the wall, a reduction in the diameter of the axial hole in which the resistor is disposed may be considered.
However, as the diameter of the axial hole decreases, the outer diameter of the resistor to be disposed within the axial hole also decreases. As a result, in the resistor, an electrical load per unit area increases, which is more likely to cause the losses of conductive paths. Since the reduction in diameter accompanies a reduction in the number of conductive paths in the resistor, even if a relatively small number of conductive paths are lost, resistance may increase sharply. That is, if the size of a spark plug is merely reduced without taking any measures, it may lead to the failure of spark discharge (i.e., misfire) at a relatively early stage.